Formation of Ions
- An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
- This loss or gain of electrons takes place to obtain a full outer shell of electrons
- The electronic structure of group 1, 2, 6 and 7 ions will be the same as a noble gas - such as helium, neon, and argon
- Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons
- Positive ions are called cations and form when atoms lose electrons, meaning they have more protons than electrons
- All metals lose electrons to other atoms to become positively charged ions
- All non-metals gain electrons from other atoms to become negatively charged ions
Diagram to show the formation of a sodium ion
A sodium atom loses its outer shell electron to form a positive ion
Diagram to show the formation of a chloride ion
A chlorine atom gains an electron to form a chloride ion
- The positive and negative charges are held together by the strong electrostatic forces of attraction between oppositely charged ions
- This is what holds ionic compounds together
Dot & Cross Diagrams
- These show the arrangement of the electrons in an ionic compound
- The electrons are shown as dots and crosses
- The charge of the ion is spread evenly which is shown by using brackets
- The charge on each ion is written at the top right-hand corner
Dot and cross diagram of sodium chloride
Sodium transfers its outer electron to chlorine
Exam Tip
The number of electrons that an atom gains or loses is the same as the charge.
For example, if a magnesium atom loses 2 electrons, then the charge will be 2+, if a bromine atom gains 1 electron then the charge will be 1-.
When drawing dot and cross diagrams, make sure you use the dot and cross notation so that it is clear which electrons have been transferred.
